It is well known to form an image by phase contrast imaging methods in which phase modulation of light is converted into intensity modulation. As opposed to intensity modulation, phase modulation does not involve loss of energy.
A generalized phase contrast imaging method and system for synthesizing a prescribed intensity pattern is disclosed in WO 96/34207, which is hereby incorporated by reference. The generalized method is not based on the so-called Zernike approximation that the phase shift φ is less than 1 radian. An improved method is provided without this approximation and based on imaging with a simple one-to-one mapping of resolution elements or pixels of a spatial phase modulator and resolution elements of the generated intensity pattern.
The disclosed phase contrast imaging method of synthesizing a prescribed intensity pattern I(x′,y′), comprises the steps of:                dividing the intensity pattern I(x′,y′)=|(x′,y′)|2 into pixels in accordance with the disposition of resolution elements (x,y) of a spatial phase mask having                    a plurality of individual resolution elements (x,y), each resolution element (x,y) modulating the phase of electromagnetic radiation incident upon it with a predetermined phasor value eiφ(x,y),                        radiating electromagnetic radiation towards the spatial phase mask,        Fourier or Fresnel transforming the modulated electromagnetic radiation,        phase shifting with a spatial phase filter (SPF) in a region of spatial frequencies comprising DC in the Fourier or Fresnel plane, the modulated electromagnetic radiation by a predetermined phase shift value θ in relation to the remaining part of the electromagnetic radiation, and        forming the intensity pattern by Fourier or Fresnel transforming, respectively, the phase shifted Fourier or Fresnel transformed modulated electromagnetic radiation, whereby each resolution element (x,y) of the phase mask is imaged on a corresponding resolution element (x′,y′) of the image,        calculating the phasor values eiφ(x,y) of the phase mask and the phase shift value θ in accordance witho(x′,y′)=eiφ(x′,y′)+ α(eiθ−1)        for selected phase shift values θ, α being the average of the phasors eiφ(x,y) of the resolution elements of the phase mask,        selecting, for each resolution element, one of two phasor values which represent a particular grey level, and        supplying the selected phasor values eiφ(x,y) to the resolution elements (x,y) of the spatial phase mask.        
In one embodiment disclosed in WO 96/34207, the spatial phase mask is positioned at the front focal plane of a lens while the spatial phase filter is positioned in the back focal plane of the lens, whereby a first electromagnetic field at the spatial phase mask is Fourier transformed by the lens into a second electromagnetic field at the spatial phase filter. Thus, specific spatial frequencies of the first electromagnetic field will be transmitted through the spatial phase filter at specific positions of the phase filter. For example, the energy of the electromagnetic radiation at zero frequency (DC) is modified and transformed onto the intersecting point of the Fourier plane and the optical axis of the lens also denoted the zero-order diffraction region by the phase filter.